Direct Development in Rostanga arbutus (Angas) (Mollusca: Nudibranchia) and the effects of temperature and salinity on embryos reared in the laboratory

Abstract

Rostanga arbutus from New South Wales is a small, strawberry-coloured, intertidal, dorid nudibranch which lays between 150 and 700 eggs per transparent egg ribbon. Zygotes are orange-red and approximately 184 μm in diameter. Development is direct (or ametamorphic Type 3 as defined by Bonar, 1978). Hatching occurs 15 days after oviposition at 20–21°C. Embryos pass through a reduced veliger stage during morphogenesis into benthic juveniles (approximately 234 μm in length). During the veliger stage, embryos cannot retract into their shells and they do not have an operculum, a functional velum with locomotory cilia or pedal glands. In embryos from different populations, the shell is either cap-like or type 1 (as defined by Thompson, 1961). Embryos produced by adults from Long Reef have a narrow range of tolerance over 20 temperature-salinity combinations (10, 15, 20, 25, 30°C and 20, 27, 34, 40%˚), all of which are within the ranges that occur naturally in the field. Tolerance to temperature is widest (20to 25°C) at 34%˚ salinity. Rate of development to hatching increases with temperature over the range 20 to 25°C. Both hyposaline (20, 27%˚) and hypersaline conditions (40%˚) increase the percentage of abnormally developing embryos. If embryos are exposed, even briefly, to temperatures near 13 or 32°C in conjunction with salinities ranging from 27 to 40%˚, at any time between oviposition and later stages of cleavage, they are unlikely to survive. These results correspond well with areas in which egg masses are found in the lower and middle reaches of the shore during summer and autumn (December to May), where conditions are less extreme than higher up the shore.